The present invention relates to an abnormal condition detective circuit which detects an abnormal drop of an input voltage of a switching regulator of a flyback system.
As control devices or electric machinery become smaller and lighter, simplified switching regulators are desirable to produce a power supply which is also smaller and lighter. Particularly, many switching regulators of the flyback system such as shown in FIG. 1 are employed.
In FIG. 1, a base current I.sub.B is delivered to a transistor 1 from an input dc voltage V.sub.in source through a resistor 2. When the transistor 1 is turned on, a current serving as a collector current I.sub.c flows through a winding N1 of a transformer 4 in a direction as shown. As a result, the iron core of the transformer 4 is subjected to excitation, thus producing an induced voltage across winding N2 and N3. As far as the winding N2 is concerned, no current flows therethrough in a direction of the induced voltage produced when the transistor 1 is turned on, with the result that only excitation of the iron core is conducted.
On the other hand, in the circuit including the winding N3, the induced voltage produced when the transistor 1 is turned on is applied to a differentiating circuit comprising a capacitor 5 and a resistor 6, thus allowing an instantaneous current to flow in a direction which increases the base current of the transistor 1.
As shown in FIG. 2, when the transistor 1 is turned on at time t.sub.1, the current I.sub.C increases in proportion to time. However, at time t.sub.2 determined by various circuit constants, the current I.sub.C decreases, so that an induced voltage is produced across the winding N3 in a direction opposite to that when the transistor 1 was turned on. As a result, the base current of the transistor 1 is rapidly decreased by the differentiating circuit comprising the capacitor 5 and the resistor 6.
Thus, the current I.sub.C is further decreased, so that the transistor 1 is momentarily turned off. As a result, a voltage is produced across the winding N1 in an opposite direction by magnetic flux stored in the iron core of the transformer 4 when the transistor 1 was turned on. Simultaneously with this, a voltage is also produced across the winding N2 in a direction opposite to that when the transistor 1 was turned on, thereby charging a capacitor 7 to V.sub.out through a diode 3.
At this time, when a load current I.sub.L flows, the magnetic flux energy which has been stored in the iron core of the transformer 4 for a time period during which the transistor is in the on state is dissipated. On the basis of the dissipation time (obtained as a function of the load current I.sub.L, the input voltage V.sub.in and inductance L3 of the winding N1), the off time of the transistor 1 is determined.
Under a condition where the transistor 1 is off, when the magnetic flux stored in the iron core of the transformer 4 is decreased to a fixed value, the transistor 1 is again turned on (time t.sub.3), beginning storage of the magnetic flux in the transformer 4. Thus, the transistor 1 repeatedly performs an on-off operation. At this time, the on-off time is controlled by the load current, whereby the output voltage V.sub.out is held at a fixed voltage value.
FIG. 2 shows waveforms of signals appearing on respective components of the main circuit of the switching regulator shown in FIG. 1 wherein V3 represents a voltage appearing across winding N3, I.sub.C a current flowing through the winding N1 (a collector current of the transistor 1), and I.sub.B a base current of the transistor 1.
Namely, the switching regulator of the flyback system performs the above-mentioned operation, thereby to hold the output voltage V.sub.out constant irrespective of changes of the input voltage V.sub.in and the output current I.sub.L.
As stated above, the allowed range of the input voltage V.sub.in that the switching regulator can control is essentially determined by the circuit constants or the component performance, etc. When the input voltage becomes abnormal, the output voltage becomes abnormal accordingly. For this reason, if the input voltage V.sub.in is out of a predetermined range, it is required to announce that the voltage is in abnormal condition to the next stage of the switching regulator by using any suitable method.
Since the primary and secondary sides of the transformer 4 should be kept electrically isolated, it is not preferable to directly detect abnormal condition of the input voltage V.sub.in. Accordingly, it is generally preferable to detect an abnormal condition of the input voltage V.sub.in on the primary side on the basis of the output voltage V.sub.out on the secondary side.
In view of this, the above-mentioned switching regulator shown in FIG. 1 is provided with a detection circuit which detects the abnormal condition of the input voltage V.sub.in on the basis of the abnormal condition of the output voltage V.sub.out. In accordance with this detection circuit, the base current of a single output transistor 12 is controlled by a Zener diode 10 to output a signal indicating that the voltage is in abnormal condition.
FIG. 3 is a characteristic curve showing the relationship between the output voltage V.sub.out and the input voltage V.sub.in. The above-mentioned voltage abnormal condition detection circuit operates to detect an abnormal drop below level B of the input voltage V.sub.in due to the fact that the output voltage V.sub.out has abnormally lowered below level A.
However, this conventional voltage abnormal condition detection circuit fails to detect the abnormal condition of an output voltage obtained when the input voltage V.sub.in is between points B and C shown in FIG. 3. Accordingly, the output voltage is continuously delivered to the system of the next stage with it being in an abnormal condition between these points, resulting in the possibility that the operation of the system would become abnormal. For the purpose of allowing the points B and C to be close to each other, it is required that the level A is caused to approach the normal value of the output voltage. However, in the case where the level A is caused to approach the normal value, even if a normal input voltage such as represented by V.sub.in &gt;&gt;C could be obtained, when the V.sub.out is slightly lowered for any reason, an abnormal condition would be detected. Accordingly, the level A is required to be set to a value below the normal value to some extent.